(a) Field of the Invention
The present invention relates to improvements in top entry ball valves and particularly metal seated top entry ball valves.
Metal to metal seated ball valves are utilized in a number of applications, especially where the abrasive action of fluids and solids causes erosion of resilient seats. Even Y-pattern glove valves have a very limited life span in such instances. Apart from this, ball valves with resilient seats cannot be used in applications where temperatures exceed 400.degree.-500.degree. F.
Typical services for metal seated ball valves include; abrasive process and pipeline slurries, high pressure steam blowdown, heavy oil service, high temperature gasification systems, safety shut-off for combustion controls, isolating steam flow at pressures of 1,500 to 3,675 PSI and flow control of corrosive and erosive fluids at high pressure or temperature. Line pressures in these systems may go as high as 9,000 PSI and operating temperatures may range as low as minus 250.degree. F. (minus 157 C.) to above 1,000 F. (538.degree. C.). Metal seated trunnion mounted full bore ball valves are mostly suited for the these extreme services.
(b) Description of Prior Art
The deemed most advanced prior art designs in ball valves of the metal seated type are manufactured by the firm Neles in Finland and the firm EPG in the U.S.A. In the prior art designs, the ball and stem are one unit, the ball being trunnion mounted via a pair of trunnions mounted-guided in bearing sleeves. Such ensures reliable response and operation with little effect from hydraulic thrust of flow. The surface of the ball is usually hard faced with stellite or alternatively overlaid with chromium or carbide oxides.
In order to achieve high seat loading in valve closed position, the prior art seats, with stellite seating faces, are preloaded by springs, a single one in the case of Neles and multiply ones in the case of EPG. A major drawback regarding such designs stems from the fact the springs are in the flow area of the valve and are thus subject to fatique and breakage.
Again, in the prior art, to ensure high quality and positive sealing, the surfaces of the balls and seats are diamond lapped finished to 16-24 r.m.s. in sets. Nothwithstanding the high quality surface finishes, the operating torques are three-four times higher than ball valves having resilient seats, due to the sealing force exerted by the two spring means mentioned above. Neles therefore limits the maximum pressure for metal seated valves to ANSI class 600. EPG valves however are available in sizes 2-8 inches in ANSI classes 150-1500 and 1/2 inch-8 inches in classes 2500 and 4500 lbs. All these valves are very expensive and it must be emphasized, use powerful actuators.
Regarding leakage rates, Neles specifies standard allowable factory leakage rate at maximum rated pressure--100 cm.sup.3 /per hour/per inch of diameter of nominal valve size. For example, a two inch valve--200 cm.sup.3 /hour. EPG specifies tightness rates comparable to globe and gate valves.
A further major drawback regarding the Neles metal seated ball valves is that they are not serviceable in line, due to having symmetrical split body construction. While EPG designs include the top entry feature, allowing in-line service and replacement of parts, they include an additional flange cover on the bottom of the valve body. A further prior art design, being one manufactured by Hills McCanna, comprises a top entry standard ball valve with tapered metal seats, but includes non-trunnion free-floating ball. In this valve, the metal seats are lapped-in with the tapered body surface and the ball. There is a constant load on the body seat-ball sealing surfaces from a heavy spring again located in the flow area, and the wedge effect of the tapered seats. The full sealing load is maintained during the 90.degree. ball movement, resulting in enormous operating torques being required. There are no spring loaded seats to retract as in Neles or EPG types. The manufacturer overcomes this serious deficiency by installing on the bottom of the valve an adjusting stem-rod, using stem packing and a gasket seal. The position of ball and seats (with the cover removed) is adjusted to a zero position which allows the ball to turn. The sealing thrust is then accomplished by the spring. The valve is offered for ANSI classes 150, 300 and 600 lbs. The free-floating ball increases the breakaway torque of a closed valve and is not recommonded for flow control applications.